A prior art SBA laser was reported by Mihashi et al in the Extended Abstracts of the 17th Conference on Solid State Devices and Materials, Tokyo, 1985, pages 63-66, and is shown in FIG. 2.
FIG. 2 shows a cross-sectional view of an SBA (Self aligned laser with Bent Active layer) laser that is produced by the metal organic vapor deposition (MO-CVD) method.
In this figure, the reference numeral 1 designates a p type GaAs substrate, the reference numeral 2 designates a buffer layer comprising p type Al.sub.0.43 Ga.sub.0.57 As, the reference numeral 3 designates a current blocking layer comprising n type GaAs, the reference numeral 5 designates a first cladding layer comprising p type Al.sub.0.43 Ga.sub.0.57 As, the reference numeral 6 designates an active layer comprising undoped Al.sub.0.07 Ga.sub.0.93 As, the reference numeral 7 designates a second cladding layer comprising n type Al.sub.0.43 Ga.sub.0.57 As, the reference numeral 8 designates a contact layer comprising n type GaAs, the reference numeral 9 designates an absorbed oxygen film, the reference numeral 10 designates a stripe groove which is produced in a reverse trapezoid cross section at the current blocking layer 3. The reference numeral 11 designates an active region, the reference numerals 12 and 13 designate an n electrode and a p electrode fixed to the contact layer 8 and the substrate 1, respectively.
The method of producing this SBA laser will be described briefly.
At first, a buffer layer 2 and a current blocking layer 3 are grown on the substrate 1 by MO-CVD method in a first crystal growth. After the growth, a stripe groove 10 having a reverse trapezoid cross section is produced at the current blocking layer 3 using an etchant for selectively etching only GaAs. As a result of this selective etching, the buffer layer 2 comprising p type Al.sub.0.43 Ga.sub.0.57 As is exposed at the bottom of the stripe groove. Next, in the second crystal growth using MO-CVD method a first cladding layer 5, an active layer 6, a second cladding layer 7, and a contact layer 8 are grown successively on the wafer having the stripe groove 10. After the second crystal growth, an n type electrode 12 and a p type electrode 13 are produced on the contact layer 8 and at the surface of the substrate 1, respectively, by a method of such as vapor plating or sputtering, thereby completing a SBA laser.
In this SBA laser, a current flows through the stripe groove 10 produced at the current blocking layer 3, and the portion parallel with the substrate 1 positioned above the stripe groove 10 of the active layer 6 becomes an active region 11. Furthermore, by using a MO-CVD method, the active layer 6 can be bent in a configuration close to that of the stripe groove 10. In this bent portion, there arises a refractive index difference in the transverse direction, and thus light is efficiently confined in the transverse direction, thereby resulting in a low threshold current and a high efficiency.
In this prior art method of producing an SBA laser, a buffer layer 2 comprising p type Al.sub.0.43 Ga.sub.0.57 As is exposed at the bottom of the stripe groove 10 by the etching to form the stripe groove 10. Meanwhile, as Al has a nature to be easily combined with oxygen, a thin oxide layer is produced at the surface of the buffer layer 2 by the etching or a water washing process. Further, buffer layer 2 is exposed to the air until a second crystal growth is conducted so that oxidation proceeds with the passage of time. Generally the oxide film is not a solid one, and almost all of it is dissociated and removed when the temperature is increased for subsequent crystal growth by the MO-CVD method. However, occasionally a quite thin oxide layer remains locally, or an absorbed oxygen film 9 resulting from a high concentration of oxygen combined with aluminum, remains even if an oxide layer is not formed.
Oxygen is taken into the crystal when a first cladding layer 5 comprising p type Al.sub.0.43 Ga.sub.0.57 As is directly grown on the buffer layer 2, thereby producing a high resistance layer including oxygen which adversely affects crystallinity. Such a reduction in crystallinity and production of a high resistance layer harm the reproducibility of the laser characteristics and may increase the threshold current and operational current. Furthermore, the temperature rise of the laser during operation is increased, thereby inviting deterioration of the laser, adversely affecting reliability.